(1) Field of the Invention
This invention relates to a field-effect transistor and a method for fabricating such a field-effect transistor and, more particularly, to a field-effect transistor in which germanium (Ge) or silicon germanium (SiGe) is used as a channel material and a method for fabricating such a field-effect transistor.
(2) Description of the Related Art
Metal oxide semiconductor field effect transistors (MOSFETs) in which silicon (Si) is used as a channel material are widely used now. In recent years an attempt to improve carrier mobility by, for example, applying a stress to a channel region has been made (see, for example, U.S. Pat. No. 6,621,131). In addition, to improve carrier mobility, the development of, for example, a MOSFET in which germanium is used as a channel material is under development.
Conventional germanium channel MOSFETs in which germanium is used as a channel material have the same structure as existing silicon channel MOSFETs in which silicon is used as a channel material. In many cases, diffusion layers doped with impurities are used as source and drain regions in such germanium channel MOSFETs.
So far, however, only germanium p-channel MOSFETs are under development. If a germanium channel MOSFET is fabricated by using a Ge substrate or the like, the Ge substrate or the like is doped with n-type impurity atoms to form diffusion layers as source and drain regions. By doing so, pn junctions are formed between a channel region and the source region and between the channel region and the drain region. Usually diffusion coefficients for n-type impurities are high and their solid solubility is low. Therefore, if the above method is used, it is essentially difficult to form pn junctions between the channel region and the source region and between the channel region and the drain region. This is one reason why it was difficult to realize a very minute germanium n-channel MOSFET. As a result, minute high-speed complementary metal oxide semiconductor field effect transistors (CMOSFETs) including a germanium p-channel MOSFET and a germanium n-channel MOSFET have not been realized yet.
To increase operation speed, a method for using silicon germanium as a channel material has conventionally been examined as well. However, the same applies to such a silicon germanium channel MOSFET.